The present invention relates to the electrolytic production of metals such as aluminum. More particularly, the invention relates to electrolysis in a cell having a cermet inert anode comprising a ceramic phase and a metal phase.
The energy and cost efficiency of aluminum smelting can be significantly reduced with the use of inert, non-consumable and dimensionally stable anodes. Replacement of traditional carbon anodes with inert anodes allows a highly productive cell design to be utilized, thereby reducing capital costs. Significant environmental benefits are also possible because inert anodes produce essentially no CO2 or CF4 emissions. Some examples of inert anode compositions are provided in U.S. Pat. Nos. 4,374,050, 4,374,761, 4,399,008, 4,455,211, 4,582,585, 4,584,172, 4,620,905, 5,279,715, 5,794,112 and 5,865,980, assigned to the assignee of the present application. These patents are incorporated herein by reference.
A significant challenge to the commercialization of inert anode technology is the anode material. Researchers have been searching for suitable inert anode materials since the early years of the Hall-Heroult process. The anode material must satisfy a number of very difficult conditions. For example, the material must not react with or dissolve to any significant extent in the cryolite electrolyte. It must not react with oxygen or corrode in an oxygen-containing atmosphere. It should be thermally stable at temperatures of about 1,000xc2x0 C. It must be relatively inexpensive and should have good mechanical strength. It must have high electrical conductivity at the smelting cell operating temperatures, e.g., about 900-1,000xc2x0 C., so that the voltage drop at the anode is low.
In addition to the above-noted criteria, aluminum produced with the inert anodes should not be contaminated with constituents of the anode material to any appreciable extent. Although the use of inert anodes in aluminum electrolytic reduction cells has been proposed in the past, the use of such inert anodes has not been put into commercial practice. One reason for this lack of implementation has been the long-standing inability to produce aluminum of commercial grade purity with inert anodes. For example, impurity levels of Fe, Cu and/or Ni have been found to be unacceptably high in aluminum produced with known inert anode materials.
The present invention has been developed in view of the foregoing, and to address other deficiencies of the prior art.
The present invention provides an inert anode comprising a ceramic phase and a metal phase. The ceramic phase preferably comprises oxides of iron, nickel and at least one other metal such as zinc or cobalt. The metal phase preferably comprises at least one metal selected from Cu, Ag, Pd, Pt, Au, Rh, Ru, Ir and Os.
An aspect of the invention is to provide an inert anode composition suitable for usage in a molten salt bath. In one embodiment, the composition comprises at least one ceramic phase of the formula NixFe2yMzO(3y+x+z)xc2x1xcex4, where M is at least one metal selected from Zn, Co, Al, Li, Cu, Ti, V, Cr, Zr, Nb, Ta, W, Mo, Hf and rare earths, x is from about 0.1 to about 0.99, y is from about 0.0001 to about 0.9, and z is from about 0.0001 to about 0.5. The oxygen stoichiometry may vary by a factor of xcex4 which may range from 0 to 0.3. In this formula, the oxygen may be partially substituted with F and/or N. The cermet inert anode composition also includes at least one metal phase. A preferred metal phase includes Cu and/or Ag, and may also include at least one noble metal selected from Pd, Pt, Au, Rh, Ru, Ir and Os.
Another aspect of the invention is to provide a method of making a cermet inert anode composition. In one embodiment, the method includes the steps of mixing at least one metal with a ceramic material of the formula NixFe2yMzO(3y+x+z)xc2x1xcex4, where M is at least one metal selected from Zn, Co, Al, Li, Cu, Ti, V, Cr, Zr, Nb, Ta, W, Mo, Hf and rare earths, x is from about 0.1 to about 0.99, y is from about 0.0001 to about 0.9, z is from about 0.0001 to about 0.5, and xcex4 is from 0 to about 0.3, pressing the mixture, and sintering the mixture.
A further aspect of the invention is to provide an electrolytic cell for producing metal. The cell includes a molten salt bath comprising an electrolyte and an oxide of a metal to be collected, a cathode, and a cermet inert anode of the present invention.
Another aspect of the present invention is to provide a method of producing commercial purity aluminum, utilizing the cermet inert anode of the present invention.
Other aspects and advantages of the invention will occur to persons skilled in the art from the following detailed description.